As magenta color image-forming couplers (hereinafter referred to more simply as "magenta couplers"), various pyrazolone derivatives are known. However, these pyrazolone derivative couplers show low coloring efficiency (conversion efficiency of the coupler into a dye) when they are used for color photographic materials. Particularly, in a so-called four-equivalent coupler having no substituent at the coupling position, the coupler forms a dye to an extent of only about 1/2 mole of dye per mole of coupler.
As a coupler for improving the coloring efficiency, a so-called two-equivalent coupler, e.g., a pyrazolone type magenta coupler having a substituent introduced at the coupling active position thereof, with the substituent being split off (released) in a color development step, is known. Examples of such couplers are described in U.S. Pat. Nos. 3,311,476, 3,419,391, 3,617,291, 3,926,631, etc.
Also, magenta couplers having a substituent connected through a sulfur atom to the coupling active position of the coupler are known. Examples include couplers having a thiocyano group as described in U.S. Pat. No. 3,214,437; couplers having an acrylthio group or a thioacrylthio group as described in U.S. Pat. No. 4,032,346; couplers having an arylthio group or a heterocyclic thio group as described in U.S. Pat. Nos. 3,227,554, and 3,701,783; and further as described in Japanese Patent Publication No. 34,044/'78; and couplers having an alkylthio group as described in West German Patent Application (OLS) No. 2,944,601. Also Japanese Patent Application (OPI) No. 57,239/'84 describes a pyrazolone coupler having a substituted alkylthio group of a specific chemical structure as a releasable group (the term "OPI" indicates an unexamined Japanese patent application open to public inspection) and Japanese Patent Application (OPI) No. 57,240/'84 describes a specific magenta coupler having a releasable group which blocks sulfur atom in the alkylthio group by an intramolecular nucleophilic reaction after a coupling reaction
However, the present inventors have found that when a coupler having an arylthio group at the coupling active position thereof in the magenta couplers as described in U.S. Pat. Nos. 3,227,554 and 3,701,783 is used for a color photographic material and color images are formed, the images thus formed are insufficient in storability. In other words, when color images formed using the above-described color photographic material are stored for a long period of time, the color images are insufficient with respect to resisting change in densities; discoloring and fading by the action of light, heat, humidity, etc.; change of balance of the three colors, yellow, magenta, and cyan; and the occurrence of staining in non-colored portions.
Furthermore, it has also been found that when the magenta coupler having an arylthio group as a releasable group described in aforesaid Japanese Patent Application (OPI) No. 34,044/'78 is used for color photographic material and color images are formed using the color photographic material, the color images formed are insufficient in storability as in the above-described case.
Moreover, it can be said that the magenta coupler having an arylthio group as the releasable group described in Japanese Patent Application (OPI) No. 35,858/'82 is a significant coupler in the point of overcoming the problem of light fastness among the abovedescribed various difficulties of the conventional magenta couplers, but the color photographic materials using these magenta couplers still leave room for improvement with respect to the change in color balance after fading and the formation of stain.
Also, the conventional magenta couplers having an arylthio group as the releasable group as described above encounter the problem that the coloring property is reduced in the case of processing the color photographic materials containing the couplers with a color developer containing a salt of an alkaline earth metal such as calcium, magnesium, etc. This becomes a fatal defect in the case of preparing the processing liquid using water containing a large amount of alkaline earth metal salts, i.e., hard water. Further, in color developing laboratories around the world, it is rare to be able to use soft water for the processing liquid, and hard water is usually employed as the water for the processing liquid.
Accordingly, color photographic materials containing conventionally known magenta couplers having an arylthio group as a releasing group can be developed only in definite developing laboratories using no hard water, although the coulers have some characteristics.